1. Field of the Invention
The present invention relates to a piezoelectric acoustic component, and more particularly, to a piezoelectric buzzer or a piezoelectric receiver and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, a piezoelectric acoustic component is widely used as a piezoelectric buzzer or a piezoelectric receiver that generates an alarm sound or an operating sound in electronic equipment, household electrical appliances, or mobile telephones. This type of piezoelectric acoustic component is generally manufactured by the steps of adhering a circular piezoelectric element on one of the surfaces of a circular metal plate to provide a unimorph type diaphragm, retaining the peripheral edge of the metal plate in the circular case with silicone rubber, and closing the opening of the case with a cover.
However, the circular diaphragm has a reduced productive efficiency, and thus, the efficiency of acoustic conversion is low, and miniaturization is difficult.
Accordingly, the applicant of the present invention disclosed in Japanese Unexamined Patent Application No. 11-293204 a piezoelectric acoustic component in which a square diaphragm is used to improve the efficiency of productivity and acoustic conversion, and to enable miniaturization. This piezoelectric acoustic component includes a diaphragm having a square piezoelectric element attached on one of surfaces of the square metal plate and an insulating case having a top wall portion, four side wall portions, and a supporting portions within two opposing side walls, and a plate substrate provided with first and second external electrodes, wherein the diaphragm is mounted in the case, the opposing two sides of the diaphragm and the supporting portion are fixed by the supporting material, and the clearance between the remaining two sides of the diaphragm and the case is sealed by a resilient sealing material such that the acoustic space is defined between the diaphragm and the top wall portion of the case. Then, the end of an opening provided on the side wall of the case is adhered onto the substrate, the metal plate is electrically connected to the first external electrode, and the electrode of the piezoelectric element is electrically connected to the second external electrode.
In the currently manufactured electronic components, surface mounting using a reflow soldering method is generally used, and the components are assembled primarily by a machine. Thus, the piezoelectric acoustic component must also have a surface mounted structure. To this end, it is preferable to connect the diaphragm and the external electrode of the substrate electrically using a conductive adhesive. However, when a conventional epoxy conductive adhesive is used, sufficient performance cannot be obtained in terms of sound pressure characteristics and impact resistant properties. In other words, in mobile equipment such as a mobile telephone which is susceptible to large impact loads, for example, by dropping it on the floor accidentally, an epoxy conductive adhesive may be cracked due to the impact load, thereby disconnecting the diaphragm and the external electrode of the substrate.
To overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric acoustic component having excellent efficiencies of productivity and acoustic conversion, a greatly miniaturized size, and having excellent impact resistance properties.
According to a first preferred embodiment of the present invention, a piezoelectric acoustic component includes a square piezoelectric diaphragm having first and second diaphragm electrodes exposed on one end portion thereof and vibrating in the length bending mode, an insulating case having a top wall portion, four side wall portions, and supporting portions inside of two opposing side walls, and a substrate in the shape of a plate having first and second external electrodes thereon, wherein the diaphragm is stored within the case with the surface exposing the first and the second diaphragm electrodes facing toward the opposite side of the case from the top wall portion, the two opposite sides of the diaphragm are supported on the supporting portion with supporting material, and the clearance between the diaphragm and the remaining two sides are sealed with a resilient sealing material such that an acoustic space is defined between the diaphragm and the top wall portion of the case, the end of an opening provided on a side wall portion of the case is adhered onto the substrate, the first diaphragm electrode on the diaphragm is electrically connected to the first external electrode with a resilient conductive adhesive, and the second diaphragm electrode is electrically connected to the second external electrode with a resilient conductive adhesive.
According to a second preferred embodiment of the present invention, a piezoelectric acoustic component includes a square piezoelectric diaphragm having first and second diaphragm electrodes exposed on one of the end portions thereof and vibrating in the area bending mode, an insulating case having a top wall portion, four side wall portions, and a supporting portion inside of the four side wall portions, and a substrate in the shape of a plate having first and second external electrodes thereon, wherein the diaphragm is stored in the case with the surface exposing the first and the second diaphragm electrodes facing toward the opposite side of the case from the top wall portion, the four sides of the diaphragm are supported on the supporting portion with supporting material such that the acoustic space is defined between the diaphragm and the case, the end of an opening provided on a side wall portion of the case is adhered onto the substrate, the first diaphragm electrode of the diaphragm is electrically connected to the first external electrode with a resilient conductive adhesive, and the second diaphragm electrode is electrically connected to the second external electrode with a resilient conductive adhesive.
Another preferred embodiment of the present invention provides a method of manufacturing a piezoelectric acoustic component including the steps of providing a square piezoelectric diaphragm having first and second diaphragm electrodes exposed on one of the end portions thereof and vibrating in the length bending mode, providing an insulating case having a top wall portion, four side wall portions, and supporting portions inside of the opposing two side walls, and providing a substrate in the shape of a plate having first and a second external electrodes thereon, storing the diaphragm within the case with the surface exposing the first and the second diaphragm electrodes facing toward the opposite side of the case from the top wall portion and supporting the two opposite sides of the diaphragm on the supporting portion with supporting material, and sealing the clearance between the diaphragm and the remaining two sides with a resilient sealing material such that an acoustic space is defined between the diaphragm and the top wall portion of the case, applying a resilient conductive adhesive continuously from the first diaphragm electrode of the diaphragm to the end of an opening provided on a side wall portion of the case, applying a resilient conductive adhesive continuously from the second diaphragm electrode to the end of the opening provided on the side wall portion of the case, applying an insulating adhesive on the upper surface of the substrate or the end of the opening provided on the side wall portion of the case, adhering the end of the opening provided on the side wall portion of the case on the substrate with an insulating adhesive and connecting the first diaphragm electrode and the first external electrode, and the second diaphragm electrode and the second external electrode alternately with a conductive adhesive, and curing the insulating adhesive and a conductive adhesive simultaneously.
A further preferred embodiment of the present invention provides a method of manufacturing a piezoelectric acoustic component including the steps of providing a square piezoelectric diaphragm having first and second diaphragm electrodes exposed on one of the end portions thereof and vibrating in the area bending mode, providing an insulating case having a top wall portion, four side wall portions, and a supporting portion inside of the four side wall portions, providing a substrate in the shape of a plate having first and second external electrodes thereon, storing the diaphragm in the case with a surface exposing the first and second diaphragm electrodes facing toward the opposite side of the case from the top wall portion and supporting the four sides of the diaphragm on the supporting portion with supporting material such that the acoustic space is defined between the diaphragm and the case, applying a resilient conductive adhesive continuously from the second diaphragm electrode to an end of an opening provided on the side wall portion of the case, applying an insulating adhesive on the upper surface of the substrate or the end of the opening formed on the side wall portion of the case, adhering the end of the opening formed on the side wall portion of the case on the substrate with an insulating adhesive and connecting the first diaphragm electrode and the first external electrode, and the second diaphragm electrode and the second external electrode alternately with a conductive adhesive, and curing the insulating adhesive and a conductive adhesive simultaneously.
Since the piezoelectric element constituting the diaphragm is substantially square, the quantity of waste generated when the piezoelectric element is punched out of the green sheet is greatly reduced, and thus the material efficiency is greatly improved. Since the formation of the electrode and the polarization are performed in the state of a parent substrate, the production efficiency is greatly improved. Since the dimensions required for design are determined by the cut dimensions of the parent substrate, it is not necessary to produce a punch die for die-cutting the green sheet every time as is the case of the disc type piezoelectric element. In other words, since the types of the punch die, jig, or piezoelectric bodies used in the steps of die-cutting the green sheet to cutting the parent substrate are greatly reduced in comparison with the related art, the manufacture of the piezoelectric element is much less expensive and more efficient.
The first preferred embodiment of the present invention is suitable for a receiver. Since this preferred embodiment can be adapted to a wide range of frequencies, in addition to the resonant range, ranges other than the resonant range are also used. The opposite two sides of the substantially square diaphragm are supported on the supporting portion of the case with the supporting material, and the clearance between the remaining two sides and the case is sealed with the resilient sealing agent such that the piezoelectric element is displaced even when the vibrational energy of the diaphragm is relatively small. When a prescribed frequency signal is applied between the two diaphragm electrodes of the diaphragm, the piezoelectric element dilates and contracts in the prescribed direction, and the diaphragm is bent and deformed in the bending mode accordingly. At this time, when the diaphragm vibrates in the vertical direction with both ends fixed to the case as nodes, and the points of the maximum displacement P exist along the longitudinal centerline of the diaphragm as shown in FIG. 1B. In FIG. 1, the diaphragm of the unimorph type is shown as an example for clarity. In contrast, in the case of the diaphragm having a disc shape, the point of the maximum displacement P exists only at the center thereof as shown in FIG. 1A. In other words, the volume of displacement of the square diaphragm is much larger than that of the disc shaped diaphragm. Since the volume of displacement corresponds to energy for moving air, the efficiency of the acoustic conversion is greatly enhanced. Also, because the clearance between both ends along the width of the diaphragm are sealed with a sealing agent, which is resilient, displacement of the diaphragm is not hindered and thereby the sound pressure is not reduced. In addition, though both shorter ends of the diaphragm are fixed, the portion between both ends is freely displaced, and thus, lower frequency sound is produced in comparison with the disc-shaped diaphragm. In other words, to obtain the sound having the same frequency as the disc-shaped diaphragm, the dimensions are greatly reduced.
On the other hand, the second preferred embodiment of the present invention is suitable for a sounder or a ringer, and used in the resonant region in order to support a large volume at a single frequency. The four sides of the substantially square diaphragm are supported on the supporting portion of the case with the supporting material for providing excitation in the area-bending mode in order to increase vibration energy of the diaphragm. The area-bending mode the diaphragm is substantially rectangular, and the whole area of the diaphragm bends and vibrates in the direction of the thickness such that the area of the two diagonal lines that constitute the main surface of the diaphragm provides the largest displacement, in other words, such that the intersection of the diagonal lines provides the largest displacement.
In various preferred embodiments of the present invention, the supporting material is preferably a material that has a high Young""s modulus in the cured state and restrains the end portion of the diaphragm strongly, such as an epoxy adhesive, or a material that has a low Young""s modulus in the cured state, and that is weak in the force to bind the diaphragm and accepts the displacement of the diaphragm such as a resilient sealing agent, for example, silicone rubber.
FIG. 2 is a comparative drawing showing the relation between the dimensions of the circular diaphragm and the substantially square diaphragm and the resonant frequency. In this case as well, the diaphragm of unimorph type is used.
For comparison, PZT having a thickness of about 50 xcexcm is used as a piezoelectric element, and 42 Ni having a thickness of about 50xcexcm is used as a metal plate. The ratio between the length L and the width W of the substantially rectangular diaphragm is 1.67.
As is clearly shown in the drawing, when the frequency is the same, the square diaphragm may be reduced in dimensions (length, diameter) in comparison with the circular diaphragm. In other words, when the dimensions are the same, much lower frequency can be obtained.
In various preferred embodiments of the present invention, the case having the diaphragm fixed thereon is adhered and fixed on the substrate so as to have a plateshaped configuration. Then, the first diaphragm electrode is electrically connected to the first external electrode with a resilient conductive adhesive, and the second diaphragm electrode is electrically connected to the second external electrode with a resilient conductive adhesive to produce a completed acoustic component. By drawing the first and the second external electrode provided on the substrate to the back surface of the substrate, a surface mounted structure is obtained.
Since the conducive adhesive has resiliency, it resists cracks even when the equipment having the piezoelectric acoustic component mounted thereon is subject to a large impact load by accidentally dropping it on the floor, thereby preventing disconnection between the diaphragm electrode and the external electrode. In addition, since Young""s modulus of the conductive adhesive in the cured state is low, vibration of the diaphragm is not restrained, thus the sound pressure is not lowered.
Preferably, as in a third preferred embodiment of the present invention, a unimorph type piezoelectric diaphragm having a piezoelectric element adhered on one of the surfaces of the metal plate at the position displaced toward one of the side which is supported by the supporting portion of the case is used as a diaphragm, the electrode on one of the surfaces of the piezoelectric element exposed outside constitutes the first diaphragm electrode, an exposed portion of the metal plate is provided on the other side of the surface having a piezoelectric element of the diaphragm is adhered, the exposed portion constitutes the second diaphragm electrode, and the diaphragm is mounted to the case with the metal plate facing toward the top wall of the case. Though it is also possible to mount the diaphragm to the case with the piezoelectric element facing toward the top wall portion, it would be difficult to connect the surface electrode of the piezoelectric element to the second external electrode of the substrate because the surface electrode of the piezoelectric element and the substrate do not face each other in such a case. In contrast, when the diaphragm is fixed to the case with the metal plate facing toward the top wall portion of the case, connection between the surface electrode and the second external electrode with a conductive adhesive is easily made because the surface electrode of the piezoelectric element and the substrate face each other. Since the exposed portion of the metal plate is exposed on one side of the diaphragm, connection between the metal plate and the first external electrode is also easily made.
As in a fourth preferred embodiment of the present invention, by using a conductive adhesive having a Young""s modulus of about 1xc3x97105-2xc3x97109 N/m2 in the cured state as a resilient conductive adhesive, an excellent effect is obtained in terms of impact resistance and sound pressure characteristics. In this case, the Vickers hardness in the cured state will be about 30-100.
Preferably, as in a fifth preferred embodiment of the present invention, the supporting material that supports the two opposing sides of the diaphragm onto the supporting portion is formed of the same material as the resilient sealing agent, in other words, a resilient sealing material is applied on all the four sides of the diaphragm. Sealing the periphery of the diaphragm with a resilient sealing material prevents air from leaking and greatly improves the sound pressure characteristics.
By manufacturing a piezoelectric acoustic component according to the steps as set forth in the sixth preferred embodiment of the present invention, fixing of the diaphragm and the case, fixing of the case and the substrate, and electrical connection between the piezoelectric board and the external electrode on the substrate are performed in a smaller numbers of steps of the same types, whereby the piezoelectric acoustic component according to the first preferred embodiment of the present invention is manufactured at a greatly reduced cost.
Likewise, by manufacturing the piezoelectric acoustic component according to the steps as set forth in the seventh preferred embodiment of the present invention, the piezoelectric acoustic component according to the second preferred embodiment of the present invention is manufactured at a greatly reduced cost.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.